1. Field of the Invention
This invention relates to a fluorescent material which absorbs radiation such as X-ray and emits light, to a radiation detector using it, and to an X-ray CT scanner.
2. Description of the Related Art
X-ray CT (Computed Tomography) scanner is known as one of the X ray diagnostic equipment. This CT scanner comprises an X-ray tube which emits a fan-beam (fan shaped beam) X-ray, and an X-ray detective system in which many X ray detectors were put side by side. This device emits a fan-beam X-ray from the X-ray tube towards the X-ray detective system. Each time it emits the X-ray, the angle to the tomographic layer is changed, for example, by 1 degree, and X-ray absorption data is collected. By analyzing this data by computer, the X-ray absorptance in the position of each tomographic layer, is computed. And a picture is formed according to this absorptance.
As this X-ray detector, a detector using Xenon (Xe) gas, has been used up to now. Xenon gas is sealed in a gas chamber in this detector. When high voltage is given between arranged electrodes, and the X-ray is emitted, the X-ray ionizes Xenon gas. The current signal according to the intensity of the X-ray can be taken, and the picture corresponding to it can be formed. However, in this detector, it is required to seal high-pressure Xenon gas in a gas chamber. Therefore, a thick window is needed.
Therefore, the detection efficiency of X-ray became low, and sensitivity became low. In order to obtain CT scanner with high resolution, it is necessary to make thickness of an electrode plate thin as much as possible. However, if an electrode plate is made thin, an electrode plate will vibrate and a noise is generated by vibration from the outside.
On the other hand, the detector both with scintillator using fluorescent materials, and with silicon photo diode, is developed and is already put in practical use. As the fluorescent material, such as, CdWO4 single crystal, (Y,Gd)2O3:Eu, the ceramics composed of Pr and Gd2O2S:Pr, Ce, and F (GOS:Pr, called later), or polycrystal of oxide (GAGG:Ce, called later) with garnet structure composed mainly of gadolinium oxide, gallium oxide, and an aluminum oxide, and cerium oxide is used. In this detector, the scintillator absorbs X-ray and emits light. The X-ray is detected when a silicon photo diode detects this light. Here, the fluorescent material used as a scintillator emits light with the wavelength according to the energy level which is generated by the activator, doped to the host material. Since the detection efficiency of a silicon photo diode is high for the visible light with wavelength of 500 nm or longer, this detector can detect X-ray with high sensitivity. As a notation method of a fluorescent material, on both sides of “:”, the host material was indicated on left-hand side, and activator was indicated on right-hand side among the chemical formula. In the detector using such materials, it is easy to miniaturize a detecting element and to increase the number of channels. Therefore, a picture with higher resolution than the detector using xenon gas, can be made. Generally, what is required for such a fluorescent material are, little variation in X-ray characteristics due to the high homogeneity of the material, little degradation against radiation damage, little variation in the fluorescent characteristics against variation in the environment such as temperature variation, being easy to manufacture, little degradation against the manufacturing process, and being chemically stable without hygroscopicity.
In such an X-ray detector, the sensitivity becomes high, as the intensity of light (fluorescence intensity) emitted from the scintillator after absorption of X-ray is high. In order to make fluorescence intensity high, it is necessary to fully absorb X-ray. If this absorption is a little, X-ray which penetrates the scintillator will increase and this X-ray will serve as a noise source in the silicon photo diode, therefore, S/N ratio decreases. In order to reduce the X-ray penetrating the scintillator, it is necessary to make the scintillator thick. However, the detecting element cannot be miniaturized in this case, but its cost increases. Therefore, in order to fully absorb X-ray with a thin fluorescent material, X-ray absorption coefficient must be large. On the other hand, if the transmittance of this light in the fluorescent material is low, the light which does not reach the photo-diode, among the generated lights, will increase, therefore, fluorescence intensity falls substantially. Therefore, in order to make fluorescence intensity high, it is required for the fluorescent material used as a scintillator material, that (1) the absorption coefficient of X-ray should be high, that (2) the transmittance of the generated light should be high.
And also high resolution is required of X-ray computed tomography. Therefore, the miniaturization of a detecting element is required. In order to lessen influence by a subject moving, it is required to shorten scanning time. In this case, the storage time in one detecting element becomes short, and the X-ray dose absorbed in the storage time decreases. Therefore, especially high fluorescent efficiency (high fluorescence intensity) is required. Furthermore, in order to improve the time resolution of a detecting element, fluorescence just after X ray irradiation stops (afterglow), should be low. For the purpose, both decay time constant of the fluorescence, and the afterglow intensity, should be low. Here, the decay time constant of luminescence means a time after stopping X-ray irradiation until the fluorescence intensity becomes 1/e of the fluorescence intensity during the X-ray irradiation. Afterglow intensity means a ratio of the fluorescence intensity after a constant time passed, since the X-ray irradiation stops, to the fluorescence intensity during the X-ray irradiation. If the intensity decays completely exponentially, small decay time constant leads to weak afterglow directly. However, actually, afterglow does not decay exponentially. Therefore, in order to get a high-performance X-ray CT scanner with weak afterglow, it is necessary to use a fluorescent material with both small decay time constant and weak afterglow intensity. The fluorescence intensity, the decay time constant, and the afterglow intensity in various fluorescent materials currently used conventionally, are shown in Table 1.
TABLE 1DecayAfterglowTimeIntensityFluorescenceConstant(%)MaterialCrystalIntensity(μs)at 30 msACdWO4single1005.00.002BGd2O2S:Pr,Ce,Fpoly1803.00.01C(Y,Gd)2O3:Eu,Prpoly18010000.01DGd3Ga5O12:Cr,Cepoly1301400.01EGd3Al3Ga2O12:Cepoly170~0.50.01
Among the above materials, Gd3Al3Ga2O12:Ce (GGAG:Ce) emits light, by the allowed transition from 5d level to 4f level of Ce3+, made by activator Ce. For example, in Japanese Patent No. 2001-4753 and in Japanese Patent No. 2003-119070, polycrystal material of GGAG:Ce is shown as a scintillator material with both high fluorescence intensity and weak afterglow.
However, for recent high-performance X-ray CT scanner, it is needed to get the tomographic image with higher resolution. Therefore, storage time for a single X ray detecting element becomes shorter. Therefore, for the scintillator used for the X ray detecting element, more strict level is needed on the fluorescence intensity and the afterglow. The characteristics of the polycrystal of above-mentioned GGAG:Ce did not fit this demand, either. Therefore, high-performance X-ray CT scanner could not be obtained, because no fluorescent material with both high fluorescence intensity and weak afterglow, was found.
This invention is made in view of this problem, and an object of this invention is to provide the invention which solves the above-mentioned problem.